This invention relates to a multipurpose analog front-end circuit used, for example, in a modulating/demodulating device (hereinafter referred to as a modem) that supports both half-duplex and full-duplex communication.
An analog front end is a circuit that interfaces between an analog signaling system such as an analog telephone line and digital signal-processing circuits such as the modulating and demodulating circuits of a modem. The analog front end of a half-duplex 1200-bit/s modem conforming to CCITT recommendation V.23, for example, has a bandpass filter comprising a low-pass filter (LPF) and a high-pass filter (HPF) connected in series, the LPF having a cutoff frequency of 2300 Hz and the HPF having a cutoff frequency of 1100 Hz. The combined passband is accordingly 1100 to 2300 Hz.
The analog front end of a full-duplex modem has two bandpass filters, each comprising an LPF and an HPF connected in series. A typical modem for a personal computer supports CCITT recommendations V.22 and V.22bis for 1200-bit/s and 2400-bit/s data rates: the low-group bandpass filter has a passband of 800 to 1600 Hz, while the high-group bandpass filter has a passband of 2000 to 2800 Hz. The low-group bandpass filter is used for transmitting and the high-group bandpass filter for receiving (or vice versa), permitting communication in both directions at once.
In previous modems that supported both half-duplex and full-duplex communications (referred to hereinafter as multi-standard modems), the analog front-end circuits had three bandpass filters, each comprising an LPF and an HPF connected in series as described above. Two of the bandpass filters were used for full-duplex communications and the third bandpass filter was used for half-duplex communications. A problem is that the circuits are complex and expensive to fabricate. Thus a multi-standard modem incorporating three bandpass filters is significantly more costly than a full-duplex modem having only two bandpass filters. Even when the analog front end is integrated onto a single semiconductor chip, with three separate bandpass filters the chip becomes large in size, so fewer chips can be fabricated from a single semiconductor wafer. The result is a rise in the cost per chip, and in the cost of the modem (or other equipment) in which the chip is employed.